A Guide to the Natural World David Krogh © 2011 Pearson Education, Inc. Chapter 13 Lecture Outline Passing on Life’s Information: DNA Structure and Replication Biology Fifth Edition

© 2011 Pearson Education, Inc The Form and Function of Genes

© 2011 Pearson Education, Inc. The Rise of Molecular Biology In trying to decipher the structure of DNA, scientists were performing work in molecular biology, defined as the investigation of life at the level of its individual molecules.

© 2011 Pearson Education, Inc Watson and Crick: The Double Helix

© 2011 Pearson Education, Inc. Rise of Molecular Biology In trying to decipher the structure of DNA, Watson and Crick were performing work in molecular biology. This is the investigation of life at the level of its individual molecules. Molecular biology has grown greatly in importance since the 1950s.

© 2011 Pearson Education, Inc. Watson and Crick Watson and Crick met in the early 1950s at Cambridge University in England and set about to decipher the structure of DNA.

© 2011 Pearson Education, Inc. Rise of Molecular Biology James Watson and Francis Crick discovered the chemical structure of DNA in This event ushered in a new era in biology because it allowed researchers to understand some of the most fundamental processes in genetics.

© 2011 Pearson Education, Inc. Watson and Crick Figure 13.1

© 2011 Pearson Education, Inc. Rosalind Franklin Their research was aided by the work of others, including Rosalind Franklin, who was using X-ray diffraction to learn about DNA’s structure.

© 2011 Pearson Education, Inc. Rosalind Franklin Figures 13.3, 13.2

© 2011 Pearson Education, Inc The Components of DNA and Their Arrangement

© 2011 Pearson Education, Inc. The Structure of DNA The DNA molecule is composed of building blocks called nucleotides. Each nucleotide consists of: One sugar (deoxyribose) One phosphate group And one of four bases: adenine, guanine, thymine, or cytosine (A, G, T, or C)

© 2011 Pearson Education, Inc. The Structure of DNA The sugar and phosphate groups are linked together in a chain that forms the “handrails” of the DNA double helix. Bases then extend inward from the handrails, with base pairs joined to each other in the middle by hydrogen bonds.

© 2011 Pearson Education, Inc. The Structure of DNA In this base pairing, A always pairs with T across the helix, while G always pairs with C.

© 2011 Pearson Education, Inc. Phosphate group Sugar Bases deoxyriboseadenine (A) thymine (T) guanine (G) cytosine (C) Component molecules 1. The DNA molecule is composed of three types of component molecules: phosphate groups, the sugar deoxyribose, and the bases adenine, thymine, guanine, and cytosine (A, T, G, and C). Nucleotides 2. These three molecules link to form the basic building block of DNA, the nucleotide. Each nucleotide is composed of one sugar, one phosphate group, and one of the four bases—in this example, A. Across the strands of the helix, A always pairs with T, and G with C. The double helix 3. The sugar from one nucleotide links with the phosphate from the next to form the “handrails” of the double helix. Meanwhile, the bases form the “stairsteps,” each base extending across the helix to link with a complementary base extending from the other side. The Structure of DNA Figure 13.5

© 2011 Pearson Education, Inc. DNA Replication DNA is copied by means of each strand of DNA serving as a template for the synthesis of a new, complementary strand.

© 2011 Pearson Education, Inc. DNA Replication The DNA double helix first divides down the middle. Each A on an original strand then specifies a place for a T in a new strand, while each G specifies a place for a C, and so forth.

© 2011 Pearson Education, Inc. 1. DNA to be replicated 2. Strands separate 3. Each strand now serves as a template for the synthesis of a separate DNA molecule as free nucleotides base-pair with complementary nucleotides on the existing strands. 4. This results in two identical strands of DNA. Order of bases encodes Information for protein production. DNA Replication Figure 13.6

© 2011 Pearson Education, Inc. DNA Replication Each double helix produced in replication is a combination of one parental strand of DNA and one newly synthesized complementary strand. This is how life builds on itself.

© 2011 Pearson Education, Inc. DNA Replication Figure 13.7 old new

© 2011 Pearson Education, Inc. DNA Polymerases A group of enzymes known as DNA polymerases is central to DNA replication. These enzymes move along the double helix, bonding together new nucleotides in complementary DNA strands.

© 2011 Pearson Education, Inc. Protein Production DNA can encode the information for the huge number of proteins used by living things because the sequence of bases along DNA’s handrails can be laid out in an extremely varied manner.

© 2011 Pearson Education, Inc. Protein Production A collection of bases in one order encodes the information for one protein. A different sequence of bases encodes the information for a different protein.

© 2011 Pearson Education, Inc. DNA Animation 13.1: DNA

© 2011 Pearson Education, Inc Mutations

© 2011 Pearson Education, Inc. Mutations The error rate in DNA replication is very low, partly because repair enzymes are able to correct mistakes. When such mistakes are made and then not corrected, the result is a mutation: a permanent alteration in a cell’s DNA base sequence.

© 2011 Pearson Education, Inc. Starting DNAIncorrect base pairingMutation Point mutation 2. The next time a cell replicates its DNA, the replication repair mechanism may “fix” this error in such a way that a permanent alteration in the DNA sequence results. The original G will be replaced, instead of the wrongly added T. The result is an A-T base pair, whereas the cell started with a G-C base pair. 1. In replicating a cell’s DNA, mistakes are sometimes made, such that one base can be paired with another base that is not complementary to it (G with T in this case). Mutations Figure 13.8

© 2011 Pearson Education, Inc. Mutations Most mutations have no effect on an organism, but when they do have an effect, it is generally negative. Cancers result from a line of cells that have undergone types of mutations that cause them to proliferate wildly.

© 2011 Pearson Education, Inc. Mutations Some mutations come about in the body’s germ-line cells, meaning cells that become eggs or sperm. Such mutations are heritable: they can be passed on from one generation to another.

© 2011 Pearson Education, Inc. Mutations The gene for Huntington disease, which is expressed in nerve cells, is a heritable, mutated form of a normal gene.

© 2011 Pearson Education, Inc. Mutations Most mutations, however, come about in the body’s somatic cells, which are cells that do not give rise to eggs or sperm. Dangerous as these mutations may be, they cannot be passed along to offspring.

© 2011 Pearson Education, Inc. Mutagens Mutations can come about through the effects of mutagens: substances, such as cigarette smoke or ultraviolet light, that can mutate DNA.

© 2011 Pearson Education, Inc. Mutations and Evolutionary Adaptation Mutations have been important to evolution because they are the only means through which completely new genetic information can be added to a species’ genome.

© 2011 Pearson Education, Inc. Mutations and Evolutionary Adaptation The accidental reorderings of DNA sequences that mutations bring about can, in rare instances, produce new proteins that are useful to organisms.

© 2011 Pearson Education, Inc. Mutations Animation 13.2: Mutations

© 2011 Pearson Education, Inc. One-Gene, One-Enzyme Hypothesis Animation 13.3: One-Gene, One-Enzyme Hypothesis